And it seems the colourful Royal was only too happy to help the actress along, giving her advice to take into the role.

"She gave me lots of tips," Carter tells 9Honey.

Carter as Princess Margaret in the third season of the The Crown. (PA/AAP)

"I first of all asked if she approved of my casting because I definitely needed her on-side and she said 'yes', preferred me to 'the other option', which I thought was actually a very Princess Margaret remark (the little I was beginning to know of her). Ambivalence, basically - complementing and putting someone down at the same time.

"And then she told me to get the smoking right and to scrub up, that I had to get neater in just my general demeanour."

Carter joined other members of the new season cast on the red carpet at the World Premiere, including Game of Thrones and Outlander star Tobias Menzies, who will take on the role of Prince Philip.

Carter with the rest of The Crown's cast at the season three premiere. (PA/AAP)

Nailing the voice of the Duke of Edinburgh was key for the actor, so he tells 9Honey: "I just listened to him till I started bleeding from the ears".

"You need to sound like him but it can't be an act of mimicry because that's kind of irritating to listen to as well, so there's a balance to be hit," he added.

The Crown Season Three is available to stream on Netflix from Sunday November 17.